Air bags are a well-known means to protect occupants of vehicles in front-end collisions. The most common types of air bags are inflated irreversably and very rapidly by chemical means. A malfunction in the inflation control apparatus may trigger inflation during normal driving and cause a crash. It is desirable to provide an air bag which begins inflation at a slower rate in advance of actual collision in response to brake pressure, but is also actuable by harness tension or bumper impact, and is readily deflatable.